1. Field of the Invention
The invention relates to the conversion of synthesis gas to hydrocarbons. More particularly, it relates to the conversion of such synthesis gas to C.sub.5.sup.+ hydrocarbons particularly suitable for use as liquid motor fuels.
2. Description of the Prior Art
It is well known in the art that synthesis gas, i.e., hydrogen and carbon monoxide, can be converted to hydrocarbons in the presence of a variety of transition metal catalysts. Thus, certain Group VIII metals, particularly iron, cobalt, ruthenium and nickel, are known to catalyze the conversion of CO and hydrogen, also referred to as syngas, to hydrocarbons. Such metals are commonly called Fischer-Tropsch catalysts. While the use of nickel preferentially produces methane upon conversion of syngas, the use of iron, cobalt and ruthenium tends to produce hydrocarbon mixtures consisting of hydrocarbons having a larger carbon number than methane, as determined by a number of analytical means including mass spectrographic analysis of individual components and the boiling point curve method. At higher reaction temperatures, all Fischer-Tropsch catalysts tend to produce gaseous hydrocarbons, and it is readily feasible to select processing conditions to produce methane as the principal product. At lower temperatures, and usually at higher pressures, however, iron, cobalt and ruthenium produce hydrocarbon mixtures consisting of larger hydrocarbons. These products usually contain very long straight-chain hydrocarbon molecules that tend to precipitate as wax. Such wax material, boiling well beyond the boiling range of motor fuels, typically constitutes a significant fraction of the product produced in such catalytic conversion operations. For these reasons, therefore, Fischer-Tropsch catalysts have not been advantageously employed recently in the production of liquid hydrocarbon motor fuels, instead commonly producing either principally gaseous hydrocarbons, on the one hand, or hydrocarbons containing an unacceptably large amount of wax on the other. In addition, the gasoline range boiling hydrocarbon fraction that is produced has an unacceptably low octane number.
In light of such circumstances, efforts have been made to improve the performance of Fischer-Tropsch catalysts for use in various desired syngas conversions. For example, the Breck et al. patent, U.S. Pat. No. 3,013,990, discloses the use of zeolitic molecular sieves containing a Fischer-Tropsch catalyst as improved catalyst compositions. Thus, Type A, X and Y molecular sieves loaded with iron or cobalt are shown to be suitable Fischer-Tropsch hydrocarbon synthesis catalysts, as for the production of methanol from syngas. Also with respect to the conversion of syngas, Fraenkel et al., U.S. Pat. No. 4,294,725, teach that zeolites A and Y loaded with cobalt, incorporated by ion exchange and reduced in-situ with cadmium, serve as useful catalysts for synthesis of specific, small carbon number hydrocarbons. Those skilled in the art will appreciate that such catalyst materials tend to be relatively expensive and, in any event, do not produce hydrocarbon products advantageous for use as liquid motor fuels.
Efforts have also been made to improve Fischer-Tropsch catalyst performance by preparing intimate mixtures of Fischer-Tropsch metals, such as iron, with an acidic crystalline aluminosilicate, such as ZSM-5. The Chang et al. patents, U.S. Pat. Nos. 4,086,262, and 4,096,163, disclose such catalyst compositions employed in the conversion of synthesis gas to hydrocarbon mixture useful in the manufacture of heating fuels, aromatic gasoline, and chemical intermediates. When it is desired to convert syngas specifically to hydrocarbons boiling in the jet fuel+diesel oil boiling range, however, such an approach is not suitable, experiencing an effective limitation at C.sub.10 carbon number as was the case using ZSM-5 in methanol conversion, as disclosed in the Owen et al. patent, U.S. Pat. No. 3,969,426.
Another difficulty present in the production of liquid motor fuels, particularly those boiling in the gasoline boiling range, by the conversion of syngas in the presence of Fischer-Tropsch metal catalysts is the tendency of such Fischer-Tropsch metals to characteristically produce straight chain hydrocarbons consisting of a mixture of n-paraffins and n-olefins. The actual mixture obtained will be understood to depend upon the particular metal catalyst and the process conditions employed. In any event, the conversion product will generally contain only small amounts of mono-branched and almost no multi-branched hydrocarbons, as well as very little naphthenes and aromatics. The absence of significant amounts of branched or aromatic, i.e. cyclic, hydrocarbons in the conversion products results in such products having gasoline fractions of very low octane number. Such fractions are not suitable for use as gasoline without the addition of further, expensive refining steps. The larger n-paraffins produced in the C.sub.10 -C.sub.18 range by such metal catalysts are, nevertheless, desirable components for incorporation in jet and diesel fuels. However, the presence of some branched hydrocarbon components are also desired in such fractions to enhance the thermal efficiency of the overall process for converting raw syngas to such liquid motor fuels and to reduce the pour point of such fuels. In addition, the accompanying production of hydrocarbon products boiling above the diesel oil range constitutes a recognized economic and marketing burden adversely affecting the desired liquid motor fuel operation.
For the reasons above, the development of improved technology of the conversion of syngas to liquid hydrocarbon fuels is desired in the art. Such improved technology would desirably enable such syngas conversion to be carried out with (1) enhanced branching and aromatization as compared with the present production of predominantly n-paraffins and n-olefins, and (2) enhanced production of desired liquid motor fuels by reducing the formation of methane and/or of heavy hydrocarbon products boiling beyond the boiling range of diesel oil.
It is an object of the invention, therefore, to provide an improved process for the conversion of syngas to liquid hydrocarbon motor fuels.
It is another object of the invention to provide a catalyst composition capable of enhancing the conversion of syngas to such liquid motor fuels.
It is a further object of the invention to provide a process and Fischer-Tropsch catalyst composition for producing liquid motor fuels containing minimal amounts of methane and of heavy hydrocarbon products boiling beyond the boiling range of diesel oil.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.